Thermal energy storage system with phase change material and method of its operation

Abstract

Thermal energy storage system with phase change material and method of its operation An energy storage system (100) comprises a hot thermal energy storage medium (5′) and a cold thermal energy storage medium (4′), which are interconnected in a thermo-dynamic gas flow circuit. An energy converter with a motor/generator system (1A, 1B) is functionally connected to a compressor/expander system (2) for converting between electrical energy and thermal energy of the gaseous working fluid in the thermodynamic fluid circuit. A latent thermal energy storage working fluid is thermally connected to the gas flow circuit through heat exchanger (8) for providing a limit for the temperature in the cold TES medium (4′).

The invention claimed is: 
     
       1. A method of operating a thermal energy storage, TES, system having a thermodynamic gas flow circuit including:
 a gaseous working fluid that is not in liquid phase but maintained in gas phase throughout the gas flow circuit; 
 a first TES container, wherein the first TES container has a top and a bottom and contains a first TES medium for storing thermal energy, the first TES medium having an upper end and a lower end, 
 a second TES container, wherein the second TES container has a top and a bottom and contains a second TES medium for storing thermal energy, the second TES medium having an upper end and a lower end, 
 an energy converter for converting between electrical energy and thermal energy of the gaseous working fluid in the gas flow circuit; the energy converter comprising an electrical motor, an electrical generator, and a compressor/expander system, the compressor/expander system comprising a compressor and an expander, wherein the compressor is functionally connected to the motor for being driven by the motor during a charging period, and the expander is functionally connected to the generator for driving the generator during a discharging period; 
 
       the method comprising, during a charging period:
 interconnecting the top of the first TES container and the top of the second TES container through the compressor and the bottom of the first TES container and the bottom of the second TES container through the expander, 
 driving the compressor by the electric motor, receiving the gaseous working fluid from the top of the second TES container by the compressor, and adiabatically compressing the gaseous working fluid by the compressor for increasing a temperature of the gaseous working fluid, 
 providing the compressed gaseous working fluid into the top of the first TES container and through the first TES medium for transferring heat from the gaseous working fluid to the first TES medium during movement of the gaseous working fluid from the top of the first TES container to the bottom of the first TES container, 
 receiving the gaseous working fluid from the bottom of the first TES container by the expander and adiabatically expanding the gaseous working fluid for decreasing the temperature of the gaseous working fluid, 
 after the expansion, receiving the gaseous working fluid by the bottom of the second TES container, and by flow through the second TES medium towards the top of the second TES container transferring thermal energy from the second TES medium to the gaseous working fluid; 
 
       the method comprising, during a discharging period:
 interconnecting the top of the first TES container and the second TES container through the expander and the bottom of the first TES container and the second TES container through the compressor, 
 receiving the gaseous working fluid from the top of the first TES container by the expander and driving the generator for producing electrical power by work from the expander due to adiabatic expansion of the gaseous working fluid through the expander, 
 guiding the gaseous working fluid from the expander into the top of the second TES container and through the second TES medium for transferring thermal energy from the gaseous working fluid to the second TES medium during movement from the top of the second TES container to the bottom of the second TES container, 
 receiving and adiabatically compressing the gaseous working fluid by the compressor, 
 after the compression, receiving the compressed gaseous working fluid at the bottom of the first TES container for transfer of thermal energy from the first thermal medium to the gaseous working fluid during its flow towards the top of the first TES container; 
 
       wherein the system comprises a latent TES system including:
 a latent fluid flow path containing a latent working fluid that comprises a phase change material, 
 a heat exchanger located in the gas flow circuit between the compressor/expander system and the bottom of the second TES container, the heat exchanger separating the latent working fluid from the gaseous working fluid by a thermally conducting wall for exchange of thermal energy between the gaseous working fluid in the gas flow circuit and the latent working fluid in the latent flow path; 
 a pump for pumping the phase change material through the heat exchanger and creating turbulence in the phase change material during the transfer of the thermal energy in the heat exchanger; 
 
       wherein the method further comprises:
 during the charging period, after adiabatic expansion of the gaseous working fluid by the expander, guiding the gaseous working fluid through the heat exchanger and transferring thermal energy from the latent working fluid to the gaseous working fluid for increasing the temperature of the gaseous working fluid before receiving the gaseous working fluid by the bottom of the second TES container, 
 
       wherein the phase change material is in the form of ice slurry during operation of the system and the method comprises:
 during the discharging period, guiding the gaseous working fluid from the bottom of the second TES container through the heat exchanger and transferring thermal energy from the gaseous working fluid to the latent working fluid prior to the adiabatic compression by the compressor. 
 
     
     
       2. The method according to  claim 1 , wherein the latent working fluid comprises water as the phase change material and the method comprises forming water ice slurry during operation. 
     
     
       3. The method according to  claim 2 , wherein the latent working fluid comprises an additive for adjusting the freezing point of the water ice slurry. 
     
     
       4. The method according to  claim 2 , wherein the latent TES system comprises a first tank and second tank interconnected by the latent flow path for flow of the latent working fluid between the first tank and the second tank through the heat exchanger, wherein the first tank comprises the latent working fluid at a temperature above its freezing point, and the second tank comprises ice slurry of the latent working fluid during operation;
 wherein the method comprises during charging, pumping the latent working fluid from the first tank through the heat exchanger to the second tank in counterflow with the gas flow through the heat exchanger and transferring thermal energy from the water to the gas in the heat exchanger and forming ice slurry by the thermal energy transfer. 
 
     
     
       5. The method according to  claim 4 , wherein the temperature in the first tank is kept at ambient temperature due to heat exchange with the environment. 
     
     
       6. The method according to  claim 2 , wherein the latent TES system comprises an ice slurry tank and wherein the latent flow path is a latent flow circuit from the ice slurry tank to and through the heat exchanger and back to the ice slurry tank; wherein the method comprises pumping the latent working fluid from the ice slurry tank through the heat exchanger in counterflow with the gas flow through the heat exchanger and back to the ice slurry tank in a circuit during charging and during discharging. 
     
     
       7. The method according to  claim 1 , wherein the latent TES system is the only latent TES system in thermal connection with the gas flow circuit; and wherein the method comprises providing exchange of thermal energy between the gaseous working fluid and the latent working fluid only by heat exchange in the gas flow circuit at a position between the compressor/expander system and the bottom of the second TES container. 
     
     
       8. The method according to  claim 1 , wherein the TES containers are free from latent heat storage; and wherein the method comprises only providing sensible TES media in the TES containers. 
     
     
       9. The method according to  claim 1 , wherein the method comprises providing a further heat exchanger in the gas flow circuit between the bottom of the first TES container and the compressor/expander system, wherein the further heat exchanger exchanges thermal energy between the gas and an external fluid for changing a temperature of the external fluid. 
     
     
       10. The method according to  claim 1 , wherein the compressor and the expander are interconnected by a rotational shaft for synchronous motion, and wherein the method comprises driving the shaft by the motor during charging and driving the generator by the shaft during discharging. 
     
     
       11. The method according to  claim 1 , wherein the temperature T 0  of the latent working fluid is in a range of −20° C. and 0° C., and wherein the method comprises expanding the gas in the expander during charging to a temperature T e  of at least 15° C. lower than T 0 . 
     
     
       12. The method according to  claim 1 , wherein the method comprises raising the temperature of the gas by the compressor during the charging to a temperature above 400° C. 
     
     
       13. The method according to  claim 1 , wherein the first TES medium or the second TES medium or both are gas permeable and the method comprises creating a flow of the gas through the first TES medium or the second TES medium or both with direct contact between the gaseous working fluid and the first TES medium or the second TES medium or both. 
     
     
       14. The method according to  claim 3 , wherein the latent TES system comprises a first tank and second tank interconnected by the latent flow path for flow of the latent working fluid between the first tank and the second tank through the heat exchanger, wherein the first tank comprises the latent working fluid at a temperature above its freezing point, and the second tank comprises ice slurry of the latent working fluid during operation; wherein the method comprises during charging, pumping the latent working fluid from the first tank through the heat exchanger to the second tank in counterflow with the gas flow through the heat exchanger and transferring thermal energy from the water to the gas in the heat exchanger and forming ice slurry by the thermal energy transfer. 
     
     
       15. The method according to  claim 14 , wherein the temperature in the first tank is kept at ambient temperature due to heat exchange with the environment. 
     
     
       16. The method according to  claim 3 , wherein the latent TES system comprises an ice slurry tank and wherein the latent flow path is a latent flow circuit from the ice slurry tank to and through the heat exchanger and back to the ice slurry tank;
 wherein the method comprises pumping the latent working fluid from the ice slurry tank through the heat exchanger in counterflow with the gas flow through the heat exchanger and back to the ice slurry tank in a circuit during charging and during discharging. 
 
     
     
       17. A thermal energy storage, TES, system, wherein the system comprises a thermodynamic gas flow circuit including
 a gaseous working fluid that is not in liquid phase but maintained in gas phase throughout the gas flow circuit; 
 a first TES container, wherein the first TES container has a top and a bottom and contains a first TES medium for storing thermal energy, the first TES medium having an upper end and a lower end, 
 a second TES container, wherein the second TES container has a top and a bottom and contains a second TES medium for storing thermal energy, the second TES medium having an upper end and a lower end, 
 an energy converter for converting between electrical energy and thermal energy of the gaseous working fluid in the gas flow circuit; the energy converter comprising an electrical motor, an electrical generator, and a compressor/expander system, the compressor/expander system comprising a compressor and an expander, wherein the compressor is functionally connected to the motor for being driven by the motor during a charging period, and the expander is functionally connected to the generator for driving the generator during a discharging period; 
 
       wherein the system for a charging period is configured for:
 interconnecting the top of the first TES container and the top of the second TES container through the compressor and the bottom of the first TES container and second TES container through the expander, 
 driving the compressor by the electric motor, receiving the gaseous working fluid from the top of the second TES container by the compressor, and adiabatically compressing the gaseous working fluid by the compressor for increasing a temperature of the gaseous working fluid, 
 providing the compressed gaseous working fluid into the top of the first TES container and through the first TES medium for transferring heat from the gaseous working fluid to the first TES medium during movement of the gaseous working fluid from the top of the first TES container to the bottom of the first TES container, 
 receiving the gaseous working fluid from the bottom of the first TES container by the expander and adiabatically expanding the gaseous working fluid for decreasing the temperature of the gaseous working fluid, 
 after the expansion, receiving the gaseous working fluid by the bottom of the second heat exchanger, and by flow through the second TES medium towards the top of the second TES container transferring thermal energy from the second TES medium to the gaseous working fluid; 
 
       wherein the system for a discharging period is configured for:
 interconnecting the top of the first TES container and the top of the second TES container through the expander and the bottom of the first TES container and the bottom of the second TES containers through the compressor, 
 receiving the gaseous working fluid from the top of the first TES container by the expander and driving the generator for producing electrical power by work from the expander due to adiabatic expansion of the gaseous working fluid through the expander, 
 guiding the gaseous working fluid from the expander into the top of the second TES container and through the second TES medium for transferring thermal energy from the gaseous working fluid to the second TES medium during movement from the top of the second TES container to the bottom of the second TES container, 
 receiving and adiabatically compressing the gaseous working fluid by the compressor, 
 after the compression, receiving the compressed gaseous working fluid at the bottom of the first TES container for transfer of thermal energy from the first thermal medium to the gaseous working fluid during its flow towards the top of the first TES container; 
 
       wherein the system comprises a latent TES system, the latent TES system comprising:
 a latent fluid flow path containing a latent working fluid that comprises a phase change material and which is in the form of ice slurry during operation of the system, 
 a heat exchanger located in the gas flow circuit between the compressor/expander system and the bottom of the second TES container, the heat exchanger separating the latent working fluid from the gaseous working fluid by a thermally conducting wall for exchange of thermal energy between the gaseous working fluid in the gas flow circuit and the latent working fluid in the latent flow path; 
 a pump for pumping the ice slurry through the heat exchanger and creating turbulence in the ice slurry during the transfer of the thermal energy in the heat exchanger; 
 
       wherein the system is configured for:
 during the charging period, after adiabatic expansion of the gaseous working fluid by the expander, guiding the gaseous working fluid through the heat exchanger and transferring thermal energy from the latent working fluid to the gaseous working fluid for increasing the temperature of the gaseous working fluid before receiving the gaseous working fluid by the bottom of the second TES container, 
 during the discharging period, guiding the gaseous working fluid from the bottom of the second TES container through the heat exchanger and transferring thermal energy from the gaseous working fluid to the latent working fluid prior to the adiabatic compression by the compressor.